Because they like to have lots of different mortgage offers, and by relating them all to the bank's primary lending rate they can just make one announcement when rates change and leave everyone to work out for themselves what the actual rate is on their deal.
At the moment I think you should be able to get about P-3, although P varies between banks. HSBC's P is 7.5% at the moment so you can get mortgages which are at 4.5% for the first couple of years and a little higher later (mine goes to 4.73% I think). Plus you might be able to get some one-time incentives: HSBC recently gave me 1% of the mortgage amount as a credit on my HSBC credit card, so my effective first year rate was below 3.5%.